A light emitting diode (LED) is a semiconductor device which is capable of generating various lights through recombination of holes and electrons in a p-n junction between a p-type semiconductor and an n-type semiconductor when current is applied.
In the case of a light emitting diode device, current crowing occurs in a specific portion due to the characteristic of a wafer where p-type and n-type semiconductor materials are epitaxially grown. This results from an intrinsic property of a material having a different mobility than holes and electrons in the p-type and the n-type semiconductor materials.
A lateral light emitting diode has a MESA structure formed by performing anisotropic plasma etching on a wafer, where substrate/n-GaN/MQW/p-GaN are sequentially and epitaxially grown, to form an n-electrode. Since the etching is performed down to n-GaN, a MESA region corresponds to a non-emission region in which a light is not generated due to loss of an activation layer MQW. A MESA step causes an effect that a light traveling to the MESA is not extracted and is confined.
Mostly, a Ti/Al or Cr/Al-based electrode is widely used as an n-electrode of a lateral light emitting diode. The n-electrode is opaque, which prevents a light confined in a device from being emitting through the MESA region that is a non-emission region. Thus, light extraction efficiency is reduced.
A light of a gallium nitride light emitting diode is generated in a multi-quantum well (MQW) and exposed to the outside through gallium nitride. A critical angle is reduced due to a great difference between gallium nitride (refractive index=2.4) and air (refractive index=1). Thus, the light cannot be emitted to the outside and is totally reflected at the boundary to be confined therein. In particular, an indium tin oxide (ITO) layer (refractive index=1.9) is grown at an upper end of P—GaN as a current spreading layer to help light extraction. However, in the case of a MESA region and a side portion of an LED chip, a structure which may resolve a difference in refractive index is not formed. Thus, except for light emitted to a non-emission region and a side surface, only part of generated light is extracted, which causes light efficiency of the light emitting diode to be reduced.
To improve low light extraction efficiency, various technologies such as flip-chip structure, patterned sapphire substrate (PSS), photonic crystal technology, and anti-reflection layer have been applied. It is essential to improve light efficiency of a light emitting diode, and there is a need for continuous studies to improve the light extraction efficiency.